The present invention relates to a method for selecting a semiconductor laser. More particularly, it relates to a method for selecting a semiconductor laser of a high response speed for use in optical data communications requiring a high transmission rate and a high S/N ratio.
It is generally known that the high speed responsivity of a semiconductor laser is restricted by relaxation oscillation frequency ( refer to, for example, Ryoichi Itoh et al., "SEMICONDUCTOR LASER", published by BAIFUKAN CO., LTD., 1989, p. 268). Therefore, with semiconductor lasers adapted for optical communications of a high transmission rate, the high speed responsivity thereof serves as an important parameter and, hence, there is needed to select a semiconductor laser of a suitable relaxation oscillation frequency.
FIG. 4 illustrates a conventional method for selecting a semiconductor laser of a suitable relaxation oscillation frequency, and wherein denoted at numeral 1 is a semiconductor laser, at numeral 7 a pulse generator, at numeral 8 a bias tee, at numeral 9 an ultra-high speed light-receiving element, and at numeral 10 an oscilloscope of sampling type.
An optical output of the semiconductor laser 1 which is pulse driven by the pulse generator 7 is received by the ultra-high speed light-receiving element 9, passed through the bias tee 8 and represented as a pulse response waveform on the sampling-type oscilloscope 10. FIG. 5 illustrates a waveform as obtained in this manner. Relaxation oscillation appears in a rise portion of the pulse response waveform and is measured on the oscilloscope for its relaxation oscillation period t.sub.ro to find relaxation osillation frequency f.sub.r, which is the reciprocal of t.sub.ro.
With the conventional method for measuring the relaxation oscillation frequency of a semiconductor laser, relaxation oscillation period t.sub.ro is found from a pulse response waveform by using the sampling-type oscilloscope 10 to represent the pulse response waveform and measuring the oscillation period of the waveform thus represented on the oscilloscope 10. This results in the need of a complicated measuring system while involving problems of the measurement taking a relatively long time with degraded precision and of decreased productivity of semiconductor lasers.
The present invention has been accomplished to overcome the foregoing problems. It is, therefore, an object of the present invention to provide a method for selecting a semiconductor laser which is capable of obtaining an optimal semiconductor laser for application to optical communications with less cost and of avoiding a decrease in productivity for such a laser while contributing to mass productivity therefor.